I've been slowly building up my library of Nancy Drew and the Clue Crew books for my daughter. She's just learning to read, so I still read these to her for the time being, but she's hooked. She's converted her night stand in her room into the Clue Crew's desk, complete with a pretend computer, notebook, and pen.
She has also gotten her best friend hooked on these books, and they developed a cute trading system, so that they don't both buy the same books, and once they finish a book, they trade it with each other for books they haven't read yet!
The fun thing about these books is that they're an easy read, the mysteries and plot twists are not so complicated that a young reader or listener can't keep up, the subject matter has so far always been age appropriate as has the writing and themes (ie, no romance, nothing scary, nothing adult, etc), and there are enough clues in the story that your child might actually figure out the mystery before Nancy does (leading to an immense sense of satisfaction for your child).